• Enzymes help chemical reactions going on in the body to work well and quickly. Enzymes are biological catalysts; they help a reaction go faster without itself being changed by the reaction.
  • The chemical reactions in our body will still happen without enzymes, but will be too slow to keep us alive.
  • Enzymes can do different things. Some help break down large molecules into smaller ones and some help smaller chemicals to make large ones.

Enzymes inside cells

  • During DNA replication in mitosis or meiosis, the DNA double helix is unwound and the weak hydrogen bonds are separated by a particular enzyme.
  • As new bases line along each half, so complementary base pairings match, a different enzyme joins them together. As a result, two complete and identical DNA molecules are made. The enzymes are unchanged so can repeat the same action when needed.
  • During protein synthesis, many different reactions occur which are all catalysed by different enzymes.

Enzymes outside cells

  • Enzymes help break down large food molecules such as carbohydrates, proteins and fats as they are much too big to pass across the cell membranes of the gut wall. Digestion is the name of the process that breaks them down. Different enzymes are released into the mouth, stomach and small intestine to help digest these molecules so they can be absorbed into the cells.
  • Microorganisms and fungi also release digestive enzymes but they grow on and through the food they are digesting because they don’t have a gut. After the enzymes have digested the food, the smaller molecules are absorbed through the microorganism’s cell walls.

Enzyme action

  • Molecules that enzymes work on are called substrate molecules.
  • Enzymes work best at a particular temperature, called the ‘optimum’ temperature which in the human body, is usually around 40 degrees Celsius. However, some enzymes have adapted to work efficiently at very low temperatures, such as in ikaite rock in deep water.
  • When temperature increases, the substrate particles move faster as they have more energy. This means they are more likely to meet and react with enzymes.
  • Also, enzymes work best at the optimum pH. Most enzymes in our cells work best at pH 7 but enzymes that function in the digestive system have to be good at working at low or high pH levels.
  • When the temperature is too high or the pH is not right, the bonds holding the enzyme together break and thus make it lose its shape. This slows down the reaction.
  • The rate of reaction for an enzyme is also changed by the concentration of the substrate. With the increase of substrate concentration so does the rate of reaction, but only to a point. Beyond that concentration, the enzyme is incapable of working on the substrate any faster (all the active sites are full). Therefore, adding more substrate molecules will make no difference.

Specific enzymes

  • Enzymes only work with a particular substrate, so we say they have a specificity for their substrate. Enzymes are therefore named according to the substrate they catalyse:
    • Carbohydrases catalyse the breakdown of carbohydrates.
    • Proteases catalyse the breakdown of proteins.
  • All substrate molecules for one particular enzyme have the same 3-D shape in some part of their molecules. This suggests shape is incredibly important in enzyme reaction.
  • The substrate for a particular enzyme fits neatly into the active site of that enzyme. The active site takes a different shape in different enzymes. Since the shape of the substrate fits tightly into the hole of the active site, this model is called ‘lock-and-key’ hypothesis.
  • Changing the pH or temperature a little changes the shape of the active site, so the substrate doesn’t fit so well. Too much change will break the bonds within the enzyme and therefore can change the shape of the active site so much that is denatures the enzyme and destroys the active site.